An entirely new type of planet is made up mostly of water

You may remember learning the types of planets growing up: rocky planets, like Earth and Mars; gas giants, like Jupiter and Saturn; and ice giants, like Neptune and Uranus.

Now scientists at the Smithsonian Astrophysical Observatory (SAO) have discovered a new kind: a waterworld. The planet, named GJ1214b, is not merely covered with water like our oceans; most of it is water. “GJ1214b is like no planet we know of,” Zachory Berta, a graduate student at the SAO and lead author on the paper announcing the discovery, published online Tuesday in The Astrophysical Journal. “A huge fraction of its mass is made up of water.”

Contrary to what you might imagine, the “water” on GJ1214b is quite different from anything you’ve ever seen.”The water there is in really weird forms that we’re not used to on Earth,” Berta says. “There are substances that are like ice, but at a very high temperature, because the pressure is so high that the molecules are squeezed together. There is also this superfluid state of water that is more gaseous than the water we’re used to.”

If you have trouble picturing such an exotic alien waterworld, you’re not alone. “Frankly, I too have a lot of difficulty imagining what this would actually be like in person,” Berta says.

Located in the direction of the Ophiuchus, the planet is just 40 light years from Earth, making it a close neighbor compared to most of the stars in our galaxy. GJ1214b is 2.7 times Earth’s diameter and weighs nearly 7 times as much. The planet closely orbits a red-dwarf star every 38 hours, and has an estimated average temperature of 450° Fahrenheit.

Planets that orbit a star so closely do not typically contain any water, says Berta, so scientists believe that GJ1214b must have had an unusual history. “It couldn’t have formed that close, because all of the water would have evaporated off due to the heat,” he says. “So this planet probably had to have formed farther out, and somehow came inward.” Gravitational interactions with other planets may have pulled the waterworld closer in.

Berta and his thesis advisor, David Charbonneau, found the planet back in 2009 as part of the MEarth Project, which uses ground-based telescopes at the Fred Lawrence Whipple Observatory on Mount Hopkins in Arizona to discover habitable planets orbiting nearby red dwarf stars. “At the time, we had an inkling that this could be a waterworld,” Berta says. “We could measure the mass and the radius of the planet, so we knew its density, and the density was very low—too low to be explained by a big ball of rock.” The planet’s average density was calculated to be roughly 2 grams per cubic centimeter, far closer to water’s density of 1 g/cm3 than Earth’s average density of 5.5 g/cm3.

Still, with only limited information on the planet, the team couldn’t rule out other possibilities, such as a planet with a thick atmosphere of hydrogen and helium, which would similarly account for the low average density. But when the researchers were able to use the Hubble Telescope’s newly installed Wide Field Camera 3—specifically looking at light from the red dwarf star that traveled through the GJ1214b’s atmosphere before traveling towards us—they were able to rule out that possibility.

“If the big, puffy hydrogen envelope on the outside of the planet were there, we would see it, but we don’t,” says Berta. “So it looks like we’re dealing with the alternative—a planet with a whole bunch of water in it, and an atmosphere which is mostly water as well, which is consistent with what we see from the Hubble observations.” The research team describes the atmosphere hot and steamy.

Berta is less excited about finding the first of a new type of planet than the possibility of finding many more. “NASA’s Kepler telescope has found a numberof planets that are the same size as this one, but they’re much more distant, so it’s hard to observe those planets in more detail,” he says. “This is interesting not because it’s something that we’ve never seen before and will never see again, but because it’s sort of a type specimen for all of these.”

He believes investments in telescopes and other observational equipment will continue to pay great dividends as we continue to search the galaxy for planets. “The James Webb Space Telescope is NASA’s successor to Hubble. It will be great, because instead of just having a very rough picture of planets like these, we’ll be able to probe them more closely,” he says.

Berta is confident that within decades, astronomers will detect a smaller, cooler version of GJ1214b, which could theoretically harbor extraterrestrial life. “That’s the cool thing about astronomy,” he says. “As we continue to build better telescopes, we can find more and more.”

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